Polypropylene, in particular, propylene-ethylene block copolymer is widely used in applications where stiffness and impact resistance are required, such as automotive interior or exterior materials and components of electric products.
For example, JP-A 7-109316 discloses, for the purpose of improving processability, appearance, flexural modulus, surface hardness, impact resistance and paintability, a thermoplastic polymer prepared by a process having a step of producing a homopolymer of propylene, a step of producing an ethylene-propylene copolymer at an ethylene/propylene reaction ratio of from 30/70 to 50/50, a step of producing an ethylene-propylene copolymer at an ethylene/propylene reaction ratio of from 90/10 to 70/30 and a step of producing an ethylene-butene copolymer.
JP-A 2001-123038 discloses, for the purpose of improving see-through property, a propylene-based block copolymer composition comprising an ethylene-propylene random copolymer having an ethylene content of from 0.1 to 5% by weight and an ethylene-propylene-butene random copolymer.
JP-A 2003-327642 discloses, for the purpose of improving stiffness, hardness and moldability and also improving balance between toughness and impact resistance at low temperatures, a propylene-ethylene block copolymer comprising a polypropylene portion and a propylene-ethylene random copolymer portion composed of a propylene-ethylene random copolymer having an ethylene content of not less than 20% by weight but less than 50% by weight and a propylene-ethylene random copolymer having an ethylene content of not less than 50% by weight but less than 80% by weight.
JP-A 2004-217896 discloses a polypropylene-based resin composition containing, for improvement in gloss and linear expansion coefficient of the polypropylene-based resin composition, a component prepared by producing a polypropylene, an ethylene-propylene copolymer and an ethylene-butene copolymer by continuous multi-stage polymerization.
However, such conventional polypropylene-based thermoplastic materials have been desired to be further improved in stiffness and impact strength.
Therefore, the objective of the present invention is to propylene-ethylene block copolymers excellent in stiffness and impact strength and to provide molded articles thereof.